Question

I am making an online tool for identifying certain file types. I need to access some byte values from the file header to do this.

The user selects the file on the client machine. Somehow, I need to get the key byte values from the file, and then these are looked up in a server side database to categorize the file.

How can I read bytes from a client-side file?

I know I could have the user upload the file to the server, but these files are very large, and I only need a few bytes, so it would be slow and wasteful to upload the whole file.

Could I somehow upload part of the file? It seems it is difficult to cancel a html form upload and the file-part is not available after cancel. Is this correct?

Is it possible to read a file in javascript? I have googled this, but the answer is unclear. I have read that it is possible with a java applet, but only if the applet is signed.

Is there some other way?

Answer 1

You can use html5, but will need to fallback on flash or some other non-javascript method for older browsers.

http://www.html5rocks.com/en/tutorials/file/dndfiles/